1. Field of the Invention
The present invention generally relates to the fabrication of semiconductor materials, such as used in integrated circuits and, more particularly, to a method of reducing defects and improving the quality of silicon dioxide regions, especially such as thin oxide layers, in integrated circuit components.
2. Description of the Related Art
Silicon remains the most widely used semiconductor in the fabrication of integrated circuits (IC). One of the major reasons is an advantageous property of silicon that forms a stable oxide composition, silicon dioxide. This oxide provides a means of controlling the surface conditions of a silicon wafer on which the circuit is fabricated. Silicon dioxide can also act as a protective "mask" so that impurities can be intentionally inserted into specific regions of the silicon crystal lattice to alter conductivity characteristics and, hence, build discrete components in the silicon wafer. In other aspects of an IC device, silicon dioxide can be used as a dielectric or insulator for isolation of discrete components or elements of such components.
The basic oxidation process is the sharing of valence electrons between silicon and oxygen atoms to form four silicon-oxygen bonds. However, it is essentially impossible to grow absolutely perfect oxides. While impurity atoms are often deliberately introduced for the purpose of controlling electronic properties, impurity atoms can also be inadvertently incorporated as contaminants during the material formation or processing. Many of these contaminants are highly mobile in the oxide structure, e.g., alkali ions. In fact, contaminants are often introduced into an oxide layer from regions such as an adjacent silicon gate or subjacent silicon substrate during high-temperature manufacturing steps.
As a particular example, thin oxide regions are particularly susceptible to the effects of defects. Yet, for a metal-oxide-semiconductor (MOS) field effect transistor (FET) such as used in electrically erasable programmable read only memory (EEPROM) IC devices, it is desirable to use gate oxides in the sub-hundred Angstrom range of thicknesses. This is especially true in very large-scale integrated (VLSI) circuits; i.e., generally, those having more than 100,000 components per chip. For example, a 256,000 bit dynamic random access memory (DRAM) chip typically may have more than 600,000 components in a chip smaller than a fingernail.
Consider what happens if these contaminants are present in the thin gate oxide of an EEPROM transistor. An electric field is applied across the oxides in programming, erasing, and reading the data bit stored on the gate. The mobile contaminants can accumulate as a space-charge layer close to the silicon-oxide interface. This will cause a shift in the surface potential and, hence, will change the threshold voltage of the transistor. Moreover, on removal of the stress field, the space-charge becomes immobilized, resulting in a semipermanent change in the properties of the underlying material. Temperature changes can have the same effects. As a result, the defects impair the long-term stability of these devices.
All types of defects alter the electrical properties and operational characteristics of the IC device in which they are present. In practice, endurance of EEPROM devices is often determined by the integrity of sub-hundred Angstrom thin, gate oxides. VLSI circuits face serious reliability problems due to the quality of thin, gate oxides produced by current manufacturing techniques. (Current techniques are explained in depth in many classical texts, such as Semiconductor & Integrated Circuit Fabrication Techniques, Reston Publishing Co. Inc., copyright 1979 by the Fairchild Corporation; and VLSI Fabrication Principles, John Wiley & Sons, copyright 1983 by S. K. Ghandhi.)
Hence, there is a need for improvement in the quality of silicon dioxide layers, such as used as thin, gate oxides in FETs.